The measurement of concentrations of ionic components in various fluids is an increasingly common procedure. Some environmental testing procedures involve frequent, and sometimes continuous, determinations of the concentrations of one or more metal ions, especially ions of heavy metals. Similarly, certain medical diagnostic and treatment procedures involve frequent, and sometimes continuous, determinations of the concentrations of one or more ions in one or more bodily fluids of a patient. The necessity for better continuous testing methods has become increasingly apparent. Continuous, real time monitoring of serum potassium ion (K.sup.+) levels in blood and other bodily fluids is highly desirable, especially during heart bypass surgical procedures.
Several methods have been reported for the measurement of metal cation concentrations. Examples include detection based on ion exchange membranes; spectrophotometric and fluorometric techniques involving the presence of reagents; wet electrodes; and ionophore-based detection. Some of these are not effective in determining alkali metal ion concentrations, however. Among methods commonly used to determine alkali metal ion concentrations are those which monitor various optical properties. Of these, techniques measuring fluorescence are preferred to those based on other spectroscopic observations. Methods using fluorescence enjoy sensitivity and operational advantages rooted in the intrinsic separation of the excitation (probe) and emission (signal) wavelengths. Compounds useful for in vitro cation concentration determinations have been described in, for example, U.S. Pat. No. 4,808,539.
The use of fiber optic chemical sensors to create in vivo systems is well known. For instance, incorporation of a chemical sensor into a fiber optic waveguide such that the sensor will interact with the analyte and detect optical changes is known from U.S. Pat. No. 4,577,109. Use of a tethered pair of fluorescence energy transfer indicators as a chemical sensor in a fiber optic waveguide is known from U.S. Pat. No. 5,037,615. Use of fiber optics to monitor the signal generated by a substrate-immobilized fluorescer that is sufficiently close to an absorber substance to allow resonant energy transfer energy to occur is known from U.S. Pat. Nos. 4,929,561. 4,822,746 discloses the use of fiber optics to detect fluorescence in a system comprising fluorogenic substances in combination with light-absorbing ligands and light-absorbing complexes. Detection of fluorescence by fiber optics in a system comprising a solution containing a polymeric cationic material and a fluorescent anionic material in contact, through a semipermeable membrane, with a mobile ionophore selective toward a particular alkali metal ion is taught by U.S. Pat. No. 4,762,799.
Several fluorimetric methods that could potentially be adapted for in vivo/ex vivo use have been described. For instance, fluorescent probes consisting of rhodamine ester and merocyanine 540 as fluorophores and valinomycin as an ionophore are known. More recently, a fiber optic sensor employing 2,2-bis[3,4-(15-crown-5)-2-nitrophenylcarbamoxymethyl]tetradecanol-14, with Rhodamine-B attached as a fluorophore, to selectively complex potassium ions has been described. This device is specifically designed for in vivo use. However, it is subject to several limitations including potential out-migration of the dye-ionophore species; control of permeability (through crosslink density) must be determined at the time of polymerization; potential instability toward hydrolysis of the ester linkage; and lack of control of the net charge of the matrix which, if uncompensated, could lead to Donnan exclusion of the ions to be sensed.
Several of these methods have been beset by deficiencies in sensitivity and selectivity toward alkali metal ions at physiological concentrations, particularly in aqueous media at physiological pH. One method that overcomes some of the selectivity problems is based on the use of cryptands to selectively complex with potassium, described in, for example, DE 3202779 A1. The sensitivity of that method is limited, however, since detection is based on optical absorption. Also, the process must be carried out in an organic solvent in the presence of an organic base, thus not lending itself to continuous blood or fluid determinations. In U.S. Pat. No. 5,162,525 are described a family of fluorogenic ionophores based on a 4-methyl-coumarin moiety united with various cryptands. The [2.2.2] cryptand derivative, ##STR1## which is selective for the potassium ion, does not suffer from the aforementioned selectivity limitations and allows for potassium ion concentration determination by fluorescence.